Steel strapping is formed by slitting cold rolled steel strip into the required width and is used in a variety of applications which require a range of properties. Generally, the properties which must be considered when producing strapping are tensile strength, ductility, notch properties and work hardening. These properties are dependent on the composition of the steel and the heat treatment processes applied to the strapping.
The minimum tensile strength of steel strapping varies between 500 and 1250 MPa. Strapping having tensile strengths in the range 500 to 800 MPa is manufactured and sold by the applicant as `standard` strapping and strapping having tensile strengths in excess of 800 MPa is manufactured and sold by the applicant as `super` strapping.
Standard strapping is generally formed from low carbon steels and may be used in its cold rolled and slit form without heat treatment in applications requiring moderate strength levels, for example in the securing of cardboard cartons to pallets. In some instances standard strapping is formed from medium carbon steels and is subjected to a stress relief annealing treatment or a blueing heat treatment in order to improve ductility.
Super strapping is generally formed from medium carbon steels and the strapping is subjected to heat treatment to provide the required properties. Super strapping is used in heavy duty applications requiring medium to high tensile strength and good ductility, notch properties and work hardening. Uses include unitising of steel pipe into bundles, the fastening of heavy loads to pallets and containing high density wool and cotton bales.
The conventional heat treatment process for super strapping, which is a version of the so-called Austemper process, comprises:
(a) heating cold rolled steel strapping (generally having a carbon content between 0.20 and 0.60%) to between 800.degree. C. and 900.degree., to transform the structure to austenite,
(b) fast cooling the strapping in a lead or salt bath to a temperature between 350.degree. and 500.degree. C., to initiate transformation from austenite to bainite,
(c) air cooling the strapping for a short period of time to allow transformation of any remaining austenite, and
(d) quenching the strapping to ambient temperatures.
It is known that bainite has acceptable properties for medium to high tensile strength strapping. However, the Austemper process has a number of disadvantages.
First, there is a substantial capital cost associated with the use of lead, as well as costs to replace lead lost through oxidation and lead "drag-out" on the strip, costs associated with loss of product due to intermittent lead contamination of the strip, cost of maintenance of the lead baths and costs associated with minimising environmental and health problems generally associated with lead.
Second, the speed of the heat treatment process is limited by the cooling power of the lead bath and the need to allow sufficient time at the transformation temperature range for transformation of austenite to bainite. The required increase in the length of the lead quench bath necessary to allow sufficient time at the quench temperature to enable complete transformation of austenite to bainite at higher speeds would be cost prohibitive.
A third disadvantage is associated with the need to use sufficiently high carbon and manganese levels to avoid martensite formation during heat treatment with the countervailing requirements to keep the analysis lean to minimise steelmaking problems. In this regard, it is the desire of the steel maker to keep the carbon content of his steel as low as possible to avoid steel making problems. However, the lower the carbon content, the more difficult it is to produce bainite because the temperature at which martensite forms increases, and the Austemper process becomes less and less useful.
Another heat treatment process for producing higher strength steels, known as the Continuous Annealing line process, may appear at first sight to overcome certain of the problems associated with the Austemper process, but the process still has some shortcomings. The process involves a 25 to 40 sec heat up period, a 10 to 120 sec soaking period followed by a 0.5 to 30 sec cooling period. This process results in a dual phase ferrite/martensite steel, which requires a soaking period of at least 10 seconds for stable formation of ferrite and autenite phases which transform under fast cooling to ferrite and martensite. The major strengthening factor is the amount of hard martensite phase (15 to 60%) which may be assisted by ferrite strengtheners such as cold worked structure. A 15% structure would require other strengthening factors to achieve properties which are achieved according to the present invention.